1. the union of the nucleus and other cellular constituents of a male gamete (pollen or sperm) with those of a female gamete (ovum or egg) to form a zygote from which may develop a new plant —note in some species, fertilization may occur months after pollination —see double fertilization, megagametophyte, parthenogenesis, receptivity 2. the addition of nutrient elements to increase growth rate or overcome a nutrient deficiency in the soil This definition last updated 10/24/2008